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Thermoquad off idle hesitation++solved!++
#117333
09/08/08 03:18 PM
09/08/08 03:18 PM
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Joined: May 2003
Posts: 1,603
germany
440sat72
OP
top fuel
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OP
top fuel
Joined: May 2003
Posts: 1,603
germany
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i'm still tinkering around with my TQ. i got the idle speed up with my larger cam now by drilling the primary throttle blades 1/8" which worked fine (and made me able to lower the throttle blades so that the exposed area of the idle transfer slots is not too big).
however, i am still having a little hesitation when touching the gas. it is not a bog, more like a rough running. it only occurs when i step on the gas a little.
what i noticed is that when i touch the gas like that, the accelerator pump does not shoot fuel into the carb. i have to move the gas a little more to activate the accelarator pump. is that the way it should be?
Last edited by 440sat72; 09/14/08 05:47 PM.
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Re: Thernoquad off idle hesitation
[Re: 440sat72]
#117334
09/08/08 06:16 PM
09/08/08 06:16 PM
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Joined: Jan 2003
Posts: 9,982
Ansonia, CT
CJK440
master
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master
Joined: Jan 2003
Posts: 9,982
Ansonia, CT
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The accel pump on a TQ has a spring that is compressed by the throttle linkage at idle. This design automatically takes up any slop in the system. The pump should squirt fuel as soon as the throttle is touched. Perhaps not much of a shot at the most tiny movement but it should be responsive.
Describe "touch the throttle". Just breaking it off idle or light accel??
I wonder if your 1/8 holes were too big and you are not uncovering the transfer slots enough off idle.
2017 Contusion Blue Challenger T/A 392 M6 "BLKNBLU"
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Re: Thernoquad off idle hesitation
[Re: CJK440]
#117335
09/08/08 09:43 PM
09/08/08 09:43 PM
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Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
440trk
master
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master
Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
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Your accelerator pump setting is too low or the accelerator pump linkage has slop in it. The spring does NOT self-adjust. To adjust the pump, you must bend the linkage, or if it's "pretty close", you can get away with bending the adjusting tab at the pump. Check your pump setting. IF it is set properly, you need to fiddle with the linkage to get the slop out of it. When you move the throttle, the accelerator pump should immediately move...even just a little. If you have ANY throttle movement before the accelerator pump moves, you're going to have a lean stumble.
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Re: Thernoquad off idle hesitation
[Re: 440trk]
#117336
09/08/08 09:57 PM
09/08/08 09:57 PM
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Joined: Jan 2003
Posts: 9,982
Ansonia, CT
CJK440
master
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master
Joined: Jan 2003
Posts: 9,982
Ansonia, CT
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It does not self adjust, it does however take up any slack in the linkage.
If you can find slop in a TQ pump linkage, you got big problems.
Unlike a holley that has levers that push the pump the TQ has a spring that pushes the pump plunger, but at idle the linkage draws the plunger back, compressing the spring and "cocking" it. When the throttle is opened the spring is no longer held back and it shoots fuel.
Disconnect the linkage on a carb full of fuel and you'll see what I mean.
2017 Contusion Blue Challenger T/A 392 M6 "BLKNBLU"
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Re: Thernoquad off idle hesitation
[Re: CJK440]
#117337
09/08/08 10:37 PM
09/08/08 10:37 PM
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Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
440trk
master
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master
Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
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Quote:
It does not self adjust, it does however take up any slack in the linkage.
If you can find slop in a TQ pump linkage, you got big problems.
While I agree 99%...I've seen linkage that was not correctly adjusted, hang up and prevent movement at the pump. It allowed the rod to wiggle at the throttle linkage a little before unbinding. So getting slop in it, is possible. Yes, the pump spring will usually overcome it...but I've seen some pretty messed up linkage over the years, as many folks just weren't up on TQ tuning, and made things worse than they probably were before they "fixed" them. 
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Re: Thernoquad off idle hesitation
[Re: rjones]
#117339
09/09/08 12:09 AM
09/09/08 12:09 AM
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Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
440trk
master
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master
Joined: Jan 2003
Posts: 5,275
near Harrisburg, Pa
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Quote:
what i noticed is that when i touch the gas like that, the accelerator pump does not shoot fuel into the carb. i have to move the gas a little more to activate the accelarator pump. is that the way it should be?
No, you most likely have a float setting too low, or the accelerator pump adjustment is too high. Again, it's possible to have some binding in the linkage, so check that out too. You should get pump movement immediately, and fuel out of the squirter directly there after.
Quote:
This is a common problem with TQs and what causes it is the accelerator pump piston design. It is a two piece design with a plunger and a lip seal that fits loose on the plunger. The seal will usually have about .020 up and down play in it. What happens is when the throttle is closed the pump plunger is lifted to the top of its travel and the seal is pulled to the bottom of its groove. Now when you step on the throttle the plunger has to move .020 before the pump squirt will begin.
The problem is easy to cure by placing a tight o-ring around the bottom of the plunger forcing the lip seal tight against its upper sealing surface. The catch is you can't do this without removing the plunger which requires removal of the check valve at the bottom of the pump piston bore. Removal almost alway destroys it and now you have to buy a carb kit. However this is a easy mod if you have plans to rebuild the carb. Hope this helps.
Roger
Interesting observation... I can't say I've ever had a problem with one, but I can see where that could possibly occur. I would think that by setting the pump and float properly, you basically pre-load the plunger though, wouldn't you? i.e. the fuel in the bowl would be just a shade higher than the plunger setting, thereby assuring the accelerator well was always full of fuel. If So, wouldn't that effectively push the plunger up, pre-loading it against the spring assembly? Now I'm going to have to play around with one, to see what happens....
I've always had good luck setting the floats at 1" and the accelerator pump at spec for the carb. 
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Re: Thernoquad off idle hesitation
[Re: 440trk]
#117340
09/09/08 06:21 AM
09/09/08 06:21 AM
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Anonymous
Unregistered
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Anonymous
Unregistered
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thats a standard feature of thermobogs
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Re: Thernoquad off idle hesitation
#117341
09/09/08 07:10 AM
09/09/08 07:10 AM
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Anonymous
Unregistered
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Anonymous
Unregistered
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This is an interesting thread and I in no way wish to jump someone else's thread but while we're on the subject.
I have a speed demon doing the exact same thing for a while now. I have performed various suggested solutions from people local to me as well as moparts members in the past. Change float levels, change squirters, change pump settings. Nothing has really helped the stumble. I was playing with adjustments with Hemi Jimbo the other day and he offered a possible solution since everything else seems to be correct. He wondered if maybe there was a tiny leak in the pump diaphragm. Since I have been hearing some issues with Barry Grants QC dept. lately I thought maybe he is right? So I ordered a new diaphragm. It just came the other day have not had a chance to install it yet. I hope this finally works that stumble is beyond annoying. As I know this is different than the TQ carb. Do any of you guys have any input related to my issues?
Thanks.
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Re: Thernoquad off idle hesitation
#117342
09/09/08 07:43 AM
09/09/08 07:43 AM
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Joined: Jan 2003
Posts: 9,982
Ansonia, CT
CJK440
master
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master
Joined: Jan 2003
Posts: 9,982
Ansonia, CT
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If the diaphram in your demon was leaking, it would drip out of the pump cover and onto your manifold.
2017 Contusion Blue Challenger T/A 392 M6 "BLKNBLU"
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Re: Thernoquad off idle hesitation
[Re: mopowergtx]
#117345
09/09/08 09:08 PM
09/09/08 09:08 PM
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Joined: Jul 2007
Posts: 85
Midland, Tx
rjones
member
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member
Joined: Jul 2007
Posts: 85
Midland, Tx
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It is difficult to describe in just words about the accel pump piston so I thought a couple pictures might help. The first picture shows a .016 feeler gauge that demonstrates the slack that is present between the lip seal and the plunger. This slack exists when the throttle is closed because the lip seal drags on the bore when the piston is lifted. The plunger now has to take up this slack before the accel pump squirt will begin.  Here is a picture with the o-ring installed on the plunger. This o-ring is too loose but it demonstrates the idea. The o-ring just needs to be a snug fit.  If my memory is correct very early TQs had leather accel pump pistons. On these no slack is present in the pump piston and they squirt immediately when the throttle is opened. These were better designs and the only reason I can figure they changed them was to save money in production costs. However the newer one will work just as well if you are willing to o-ring it. Hoped this helps. Roger
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Re: Thernoquad off idle hesitation
[Re: rjones]
#117347
09/09/08 11:25 PM
09/09/08 11:25 PM
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Joined: Nov 2004
Posts: 1,760
Port Alberni, British Columbia
MoparDonny
top fuel
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top fuel
Joined: Nov 2004
Posts: 1,760
Port Alberni, British Columbia
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I always thought my off idle stumble with my old TQ was from a worn out primary throttle shaft bore. Just another stick in the spokes idea.  I could here a slight vacuum leak from the shaft if I gently touched the throttle, could even hear it while driving the truck if I did it just right. Don.
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Re: Thernoquad off idle hesitation
[Re: 440sat72]
#117351
09/12/08 08:48 PM
09/12/08 08:48 PM
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Joined: Jan 2003
Posts: 5,270
Missouri
MY340
master
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master
Joined: Jan 2003
Posts: 5,270
Missouri
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What size squirters are you using? Have you tried a larger squirter to see if it helps or eliminates the hesitation?
1970 FE5 Duster 360/904/3.91's SOLD
1973 TB3 SpaceDuster 340/4spd/4.10's SOLD
Moparless for now but when the opportunity is right I'll have another one.
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Re: Thernoquad off idle hesitation
[Re: rjones]
#117352
09/14/08 05:49 PM
09/14/08 05:49 PM
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Joined: May 2003
Posts: 1,603
germany
440sat72
OP
top fuel
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OP
top fuel
Joined: May 2003
Posts: 1,603
germany
|
Quote:
It is difficult to describe in just words about the accel pump piston so I thought a couple pictures might help. The first picture shows a .016 feeler gauge that demonstrates the slack that is present between the lip seal and the plunger. This slack exists when the throttle is closed because the lip seal drags on the bore when the piston is lifted. The plunger now has to take up this slack before the accel pump squirt will begin.
Here is a picture with the o-ring installed on the plunger. This o-ring is too loose but it demonstrates the idea. The o-ring just needs to be a snug fit.
If my memory is correct very early TQs had leather accel pump pistons. On these no slack is present in the pump piston and they squirt immediately when the throttle is opened. These were better designs and the only reason I can figure they changed them was to save money in production costs. However the newer one will work just as well if you are willing to o-ring it. Hoped this helps.
Roger
Roger, you are my hero! modifying the accelerator pump as you suggested cured the off idle hesitation! 
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